Fine-tuning CLI adds tg ft model-limits and download-tokenized-dataset commands

Two new fine-tuning commands are available:

• tg ft model-limits <model> prints a model's fine-tuning constraints, including sequence-length, batch-size, and LoRA rank limits.
• tg ft download-tokenized-dataset <ft_id> downloads the tokenized dataset a job trained on, so you can audit exactly what the model saw.

See the fine-tuning CLI reference.